This invention relates to analog wave filter devices of a type comprising an active device and first and second RC filter elements each providing a series resistance and a shunt capacitance, each filter element having first and second connection points between which its series resistance is defined and a third connection point for the shunt capacitance, the first connection point of the first filter element serving as an input of the filter device, the second connection point of the first filter element being connected to the first connection point of the second filter element, the second connection point of the second filter element being connected to an input of the active device, the third connection point of the first filter element being connected to an output of the active device, and the third connection point of the second filter element being connected to ground.
An analog wave filter device of the above type is known from the article in Electronics, Apr. 10, 1959, at pages 68-70. In the device described in this article the active device is a cathode follower and each filter element is composed of a descrete resistor and a discrete capacitor. It is also known from British patent specification No. 1 211 574 that in a filter device of the above type the RC filter elements can be formed by distributed resistance/capacitance components.
The present invention is concerned with the implementation of a low pass RC active filter device of the type set forth above using integrated circuit technology. The filter device to be implemented is required to serve as an anti-alias pre-filter having a low pass band corresponding to a frequency band of interest for accepting a baseband signal which is to be applied to a switched-capacitor filter device. Such an anti-alias pre-filter is required to reject noise signals of frequency above the low pass band to which the switched-capacitor filter device would otherwise respond to produce spurious output signals that "alias" bone fide output signals as produced by the switched-capacitor filter device in response to the baseband signal.
There has been recent activity in the development of RC active filter devices using integrated circuit technologies, such as MOS integrated circuit technology. Examples of this activity are given in U.K. Patent Application GB No. 2 077 469 A and European Patent Application No. 0 013 173. In each of these Applications, RC active filter devices are described having integrated operational amplifiers as active devices and integrated resistance/capacitance filter elements which are in distributed form.
The present invention proposes an integrated low pass RC active filter device of the type set forth above in which the first and second RC filter elements are in distributed form. As will be explained later in the specification, it is advantageous, particularly when using MOS integrated circuit technology, to implement these RC filter elements in this way. However, it has been found with this implementation that high frequency components in an input signal applied to the input of the filter device tend to leak directly to the output. This undesirable feedthrough effect increases with increase in frequency.
The present invention also proposes an integrated low pass RC active filter device of the type set forth above in which the active device is an operational amplifier, that is a differential amplifier with high gain and external feedback for gain-bandwidth control. Ideally, the operational amplifier should have infinite input impedance and zero output impedance, but in practice these input and ouput impedances have significant finite values, particularly when using MOS integrated circuit technology. For input signals of frequency in the low pass band, the gain-bandwidth control afforded by the external feedback maintains the amplifier output impedance at a suitably low value. However, outside the pass band the amplifier has a nonlinear behavior in that its gain rolls off with increasing frequency so that the feedback action becomes less effective and the amplifier output impedance increases. This results in an increased tendency for high frequency input components to be fed through to the filter device output.
The cumulative result of the two high frequency effects discussed above is that the filter device attenuation is progressively reduced outside the required pass band with increase in input signal frequency, once these high frequency effects start to occur. Thus, the use of the filter device as an anti-alias pre-filter as mentioned previously poses a problem if high frequency noise signals cannot be adequately attentuated by the filter device.